Fault Tree Analysis (FTA) is a method of expanding a failure event to a logical sum (OR) or a logical product (AND) of a lower event causing that to have a tree structure (hereinafter referred to as “FT diagram”), of extracting a significant cause from the lower event, and of reviewing its design in order to prevent occurrence of a fault. Since generation of an FT diagram requires an extensive knowledge and a high degree of specialization in that field, a technology for aiding generation of an FT diagram is in demand (JP2009-289020A).
FIG. 19 is an example of an FT diagram. This example analyzes a failure event in which “loss of transmission by pulley is large”. Since this failure event occurs if “slip amount is large” or if “friction force is large”, the event that “slip amount is large” and the event that “friction force is large” are lower events of the failure event, and their relationship is a logical sum.
Since the event that “friction force is large” occurs if “reaction force is large” or if “friction coefficient is large”, the event that “friction force is large” and the event that “friction coefficient is large” are lower events of the event “friction force is large”, and their relationship is a logical sum. Since the event that “reaction force is large” occurs if “belt tension is large”, the event that “belt tension is large” is a lower event of the event that “reaction force is large.”
In these events, the events that “slip amount is large”, “belt tension is large”, and “friction coefficient is large”, each not having a lower event, are called fundamental events, and in order to prevent occurrence of a failure event, it is necessary to examine a measure against these fundamental events.
In this example, a higher event and a lower event are only connected by a ruled line, and whether the lower event on the same rank is a logical sum or a logical product is not described. This is because, the lower events on the same rank are logical sums in many cases and thus, only between the higher event and the lower event and between the lower events on the same rank are connected by ruled lines in the case of the logical sum, and in the case of a logical product, the term “AND” indicating the logical product is described beside the ruled line connecting the higher event and the lower event to each other.